2.1. VIRAL VECTORS
Viral vectors permit the expression of exogenous genes in eukaryotic cells, and thereby enable the production of proteins which require postranslational modifications unique to animal cells. Viral expression vectors (reviewed in Rigby, 1983, J. Gen. Virol. 64: 255-266) have been developed using DNA viruses, such as papovaviruses (i.e. SV40 ) , adenoviruses, herpes viruses, and poxviruses (i.e. vaccinia virus, Mackett et al., 1982, Proc. Natl. Acad. Sci. U.S.A. 79:7415-7419; Panicoli et al., 1982, Proc. Natl. Acad. Sci. U.S.A. 79:4927-4931) and RNA viruses, such as retroviruses.
In disclosing the construction and applications of a murine retrovirus shuttle vector, Cepko et al. (1984, Cell 37:1053-1062) cites several properties which may be desirable in a mammalian gene transfer system, including the ability of the vector to be introduced into a wide range of hosts and the recoverability of transferred sequences as molecular clones (i.e. a vector which can "shuttle" between animal and bacterial cells; see DiMaio et al., 1982, Proc. Natl. Acad. Sci. U.S.A. 79:4030-4034). As efficient shuttle vectors, retroviruses have become a popular vehicle for transferring genes into eukaryotic cells. Retrovirus packaging cell lines (Mann et al., 1983, Cell 33:153-159; Watanabe and Temin, 1983, Mol. Cell. Biol. 3:2241-2249; Cohn and Mulligan, 1984, Proc. Natl. Acad. Sci. U.S.A. 81:6349-6353; Sorge et al., 1984, Mol. Cell. Biol. 4:1730-1737) allow production of replication-defective retrovirus vectors in the absence of helper virus; the defective retroviral vectors are able to infect and integrate into cells but cannot replicate. The ability to produce helper-free stocks of defective retroviruses using packaging cell lines protects against spread of the recombinant virus, and avoids possible dissemination of recombinant virus-induced disease. However, some retrovirus packaging lines have been shown to produce only low titers of retroviral vectors, or produce high levels of helper virus; furthermore, some retroviruses exhibit limited host ranges (Miller and Baltimore, 1986, Mol. Cell. Biol. 6:2895-2902). The recognition of human retroviruses over the past decade as the etiologic agent of Acquired Immunodeficiency Syndrome (AIDS) and some cases of T-cell and hairy cell leukemia, and the numerous examples of oncogenic animal retroviruses, have created an awareness of health risks potentially associated with the use of retrovirus vectors, particularly relevant to future prospects in human gene therapy. Many of the alternative viral vectors currently available either do not integrate into host cells at high frequency, are not easily rescuable from the integrated state, are limited in their host range, or include other viral genes, thereby creating a need for the development of a safe and efficient viral vector system.